Conventionally, with respect to a vessel, a waste heat recovery system for the vessel which generates steam at high temperature and high pressure by recovering waste heat from a main machine configured from a diesel engine, etc. has been publicly known. The generated steam is utilized as power source for generating electric power, or is supplied to a steam utilization device that is mounted on the vessel. With respect to such waste heat recovery system for the vessel, a technique of enhancing the waste heat utilization efficiency by absorbing fluctuation in steam volume by utilizing an excess steam for precooling a refrigerator etc. so as to efficiently utilize excess steam that fluctuates corresponding to usage conditions of the steam utilization device has been known. Such explanation can be seen in Patent Literature 1.